A TV SITCOM TAKES TO THE STREETS

By LISA BELKIN

Published: September 12, 1987

At the corner of 158th Street and Broadway earlier this week, in the middle of a downpour, a woman got out of a cab, then realized too late that she had left her purse on the seat. As she ran after the vehicle, a bystander joined her chase. The woman stopped, surprised, then doubled up with laughter.

The woman was Jane Curtin, star of the CBS program ''Kate & Allie,'' and she was taping a segment - the first in the show's five-year history to be set entirely out of doors. The bystander, however, was not in the script. He was but one of the added complications of taping on location.

The episode, which is scheduled to be broadcast in mid-October, is unusual for the comedy program in another way - it is a look at the very uncomical world of the homeless. Dressed in housecleaning clothes and bereft of her cash and identification because she had left them in the taxi, Ms. Curtin's character, Allie Lowell, wanders the streets looking for someone to help her get home. At the end of the show, viewers will be given the name and address of the Coalition for the Homeless, which cooperated with the producers on the segment.

But though this episode serves the beneficial purpose of focusing attention on the homeless problem, the script itself came about as a matter of chance. Last season Ms. Curtin's co-star, Susan St. James, who plays Kate McArdle, was hospitalized with a kidney stone. It looked as though she would miss the next week's taping, and the producers and writers scrambled for an idea that would not require her presence. The situation was further complicated because business trip and vacation scenarios had been used and re-used during the previous season, when Ms. St. James was on maternity leave. But when the homeless episode was proposed, it struck everyone as both timely and good drama. Street Scenes

The day after the script was agreed upon, Ms. St. James recovered. ''But the idea stuck,'' said Bill Persky, the program's director.

And because it stuck, the cast and crew spent this week on the streets of New York, following Allie as she learns what it is like to be dependent on strangers. During the taping, as the show wanders around the upper West Side, Allie sings for spare change and later is told by a pedestrian to get a job.

Along the way, New Yorkers stopped to stare at the glamorous tedium that is television production - replacing the peach tablecloths in front of Billy's Hamburger Grille with blue ones for no apparant reason; changing the yellow street signs at 97th Street and Broadway to green street signs that read 120th Street and Broadway; making sure that the ''No Parking Today'' signs put up to clear the streets do not show in the background.

This is not the first time the cast and crew have performed on location - the start of each ''Kate & Allie'' episode features an outdoor scene. But those are taped during four separate days of the year (''usually the hottest day of the summer and the coldest day of the winter,'' Ms. St. James said) and none are as complex as a full half-hour show. Onlooker Reactions

Earlier this week, when the shooting for the episode began, it was probably one of the rainiest days of the year. While the rain was heaviest, Mr. Persky and his crew set up the cameras. They began taping as soon as the downpour slowed, but by that time the wires were damp and the equipment failed to work.

Even in the rain, there were crowds. ''They were so sweet, standing there getting soaked,'' Ms. Curtin said. The personality of the crowds seemed to change as the crew moved downtown, she said. Up in Harlem, several people gave her quarters, but ''the more you move into the gentrified neighborhoods, the more impatient people get.''

Along the route, life imitated art and a handful of homeless people watched from the islands in the middle of Broadway.

The episode will coincide with the start of a national fund-raising campaign for the Coalition for the Homeless. Music written for the show will be used by the coalition as a theme song, he added.

Mr. Persky said that preparing for the episode made him more aware of the homeless problem in New York. In one scene, he said, at the beginning of Allie's story, a homeless man sits down next to her and she moves away quickly. But in another scene, he continued, toward the end of the show, another man sits next to her on another sidewalk, and she stays to talk.

''She has learned that there isn't that much difference between 'us' and 'them,' '' said Mr. Persky.